1. Field of the Invention
The present invention relates to a dosing device and, in particular, to a dosing device for demand-dosing a liquid and to a writing apparatus having such a dosing device.
2. Description of the Related Art
An electronically controlled writing apparatus is disclosed in EP 0096177. The electronic writing apparatus includes a writing tip extending into a capillary space also referred to as the secondary supply space. The small-volume secondary supply space is fluidically connected to a large-volume primary ink supply space under pressure via an ink channel, a reload valve being arranged in the ink channel. When the fluid level in the secondary supply space is too low, the reload valve is controlled in order to open for a certain time until a sufficient amount of ink has been transported from the primary supply space into the secondary supply space so that a sufficient filling level appears there. The filling level height in the secondary supply space is measured using a capacitive sensor. The capacitive sensor is embodied as a cylinder capacitor, the capacity of which depends on the filling volume of the secondary supply space. In order to be able to measure small capacity changes, a resonance circuit digitally measuring the capacity change by detuning an oscillating circuit is used. This circuit, apart from the capacity to be measured, includes an additional capacity and a quartz oscillator. Additionally, a counting mechanism is used to be able to measure the difference of the oscillating frequencies as an indication to the capacity change.
Devices for demand-dosing form a regulating system including a supply tank, a buffer reservoir and a pressure-regulating unit. The release of the liquid is from the buffer reservoir. If the pressure in the buffer reservoir sinks below a certain threshold by releasing a liquid, the pressure in the buffer reservoir is compensated by the pressure-regulating unit by transferring liquid from the supply tank into the buffer reservoir.
Such dosing devices are vacuum-regulated or negative-pressure-regulated relative to the ambient pressure so that both the buffer reservoir and the supply tank must be closed in a pressure-tight way relative to the ambient pressure. In order to be, however, able to take a liquid from a pressure-tight vessel, a gas volume must be present in this vessel. This gas volume in turn is governed by the gas laws and, for example, expands when heated, which results in a pressure increase in the vessel. Thus the negative pressure in the vessel can be reduced so that the vacuum-regulated release of liquid no longer works. A further possible reduction of the negative pressure takes place by diminishing the ambient pressure.
In addition, such dosing devices require a unilateral connection of the supply tank and the buffer reservoir to the ambient air for the purpose of a gas inlet. This connection can, in case of a failure, result in a leakage of the liquid stored.
For certain dosing devices, as are, for example, to be used in a pen, the demand-release of a liquid is important. The amount released should be able to adapt directly to the requirements of the consumer, that is in the case of a pen to its writing speed. It must be guaranteed that the liquid stream is not interrupted in a constant or changing consumption. Such a case of failure of the interruption could, for example, occur when releasing from a closed tank since when emptying it changing pressure ratios can be present and thus the liquid stream can pulse.
A further case of failure can occur by evaporating the liquid at the point of release. Solid residues from the liquid, which in the case of a pen is the ink, could plug the release opening and not allow any further release. Thus, it must be guaranteed that either no evaporation occurs or the point of release is always wetted with a liquid. The latter case of the continuous wetting requires a liquid to flow in a regulated way according to the evaporating rate.
DE 33 21 301 A1 discloses an ink supply system for writing apparatuses operating with a liquid ink, having a large volume ink space connected to a small volume ink supply space next to the writing element via a reload valve. Transferring ink from the large volume ink space to the secondary supply space takes place in a way controlled by a sensor depending on the ink volume present in the secondary ink space. As a reload valve, a hose pump which is driven in a rotating way by an electric motor and which can produce the delivery pressure required for delivering the ink to the secondary ink space is used.